Ultrafine metal powder slurry

ABSTRACT

In an ultrafine metal powder slurry containing an organic solvent, a surfactant, and an ultrafine metal powder, the surfactant is oleoyl sarcosine, the content of the ultrafine metal powder in the ultrafine metal powder slurry is 70 to 95 percent by mass, and more than 0.05 to less than 2.0 parts by mass of the surfactant is contained relative to 100 parts by mass of the ultrafine metal powder. By the above slurry, reduction in labor and treatment time can be realized in a conductive paste forming process. In addition, since aggregation of particles of the ultrafine metal powder is prevented, an ultrafine metal powder slurry can be provided having superior dispersibility and dry film density.

BACKGROUND

The present invention relates to an ultrafine metal powder slurry, andmore particularly, relates to an ultrafine metal powder slurry withsuperior dispersibility, which is used for conductive paste fillers,internal electrodes of multilayer ceramic capacitors, and the like.

An ultrafine metal powder, such as an ultrafine nickel powder, used forinternal electrodes of multilayer ceramic capacitors is a powdered highpurity metal composed, for example, of particles having an approximatelyspherical shape and a mean particle diameter of 0.1 to 1.0 μm. Anultrafine metal powder as described above is mixed with a binder such asan organic resin to form a paste for forming the internal electrodes.The paste thus formed is applied onto ceramic green sheets by screenprinting or the like to form thin films, followed by lamination ofseveral hundreds of the green sheets thus processed, so that a laminatecomposite including internal electrode layers is formed. Subsequently, amultilayer ceramic capacitor is formed by processing the above laminatecomposite through a degreasing step, a sintering step, and a firingstep. The mean particle diameter described above indicates a meanvolume-surface diameter (d3) in terms of number-size distribution.

Concomitant with the recent trend toward miniaturization and highercapacity of multilayer ceramic capacitors, it has been required that thenumber of ceramic green sheets including internal electrode layers isincreased from several hundreds to approximately one thousand. In orderto satisfy this requirement, the thickness of the internal electrodelayer is decreased from 3 μm, which has been heretofore used, to 1.5 μmor less.

In addition, when an ultrafine metal powder has poor dispersibility andincludes aggregates such as clumps, the aggregates may penetrate aceramic sheet layer to cause short circuiting of electrodes, and hencedefective units are formed. Even when the aggregates do not penetrate aceramic sheet layer, since the distance between electrodes is decreased,local current crowding occurs, thereby causing degradation and adecrease in the lifetime of a multilayer ceramic capacitor.

Accordingly, the particle size distribution D90 of an ultrafine metalpowder used as a raw material for internal electrode layers ispreferably decreased as much as possible. The term “particle sizedistribution (D90)” indicates a particle diameter at a cumulativepercentage of 90% (D90) on a volume basis.

In a related production process (Process 20 shown in FIG. 2) of anultrafine metal powder by a chemical vapor deposition (CVD) method,after residues of a metal chloride used as a raw material for theultrafine metal powder are removed for purification of the ultrafinemetal powder during Step 21, which is a wet washing step forpurification, to form a metal-water slurry, the metal powder in theslurry thus obtained is dried during Step 22, and subsequently, anultrafine metal powder product (dry powder product) is formed duringStep 23.

However, during Step 22 of drying the metal powder, since aggregationinevitably occurs due to liquid bridging forces and Van der Waals'forces generated between particles of the metal powder, there is aproblem in that the particles are not sufficiently dispersed in anorganic solvent during Step 24.

In addition, during Step 22 of drying the metal powder, since metalhydroxides are generated on the surfaces of the particles of theultrafine metal powder, the ultrafine metal powder cannot havesufficient wettability (lipophilic property) with an organic solvent. Asa result, in dispersing the dry powder obtained during Step 23 in anorganic solvent during Step 24, there is also a problem in that theparticles of the ultrafine metal powder have poor wettability with anorganic solvent aggregate with each other.

Hence, although several types of dispersion treatment, such as ball milldispersion, ultrasonic dispersion, and roll mill dispersion, areperformed in combination during Step 25, the particles of the ultrafinemetal powder processed by drying during Step 22 are liable to aggregatewith each other and have inferior dispersibility. As a result, when thedry powder processed during Step 22 is used, a paste containingapproximately up to 50 percent by mass of the ultrafine metal powder isan upper limit obtained by the dispersion treatment.

In general, the dry powder obtained during Step 23 is supplied tocustomers. Hence, the dry powder obtained during Step 23 is dispersed inan organic solvent (dispersion treatment during Step 24) at a customersite, and subsequently, viscosity adjustment is performed during Step 27as a final step, thereby forming a paste.

Accordingly, in order to disaggregate aggregated particles generatedduring Step 22 of drying the metal powder and aggregated particlesgenerated during Step 24 of dispersing the dry powder in an organicsolvent, complicated treatment such as dispersion treatment performed incombination with ball mill treatment, ultrasonic treatment, roll milltreatment, and the like during Step 25 and the filtration treatmentduring Step 26 must be additionally performed. As a result, largeamounts of labor and time are required.

Hence, the dry ultrafine metal powder product obtained during Step 23 isrequired to have superior dispersibility and to contain no aggregatedparticles.

As a technique related to a dispersion of an ultrafine metal powdercapable of satisfying the above requirements, in Japanese UnexaminedPatent Application Publication No. 2003-342607, a dispersion of a nickelpowder has been disclosed, that is prepared by adding an organic solventto a dispersion containing a water disperse medium and an ultrafinenickel powder having a mean particle diameter of 1 μm or less so as toreplace at least a part of the water disperse medium, and then adding apolar solvent for processing the nickel powder.

According to Japanese Unexamined Patent Application Publication No.2003-342607, when the dispersion of the nickel powder is prepared,treatment is preferably performed using a carbonated aqueous solution,and by treating the nickel powder in a carbonated aqueous solution,hydroxides present on the surface of the nickel powder by adhesion oradsorption are removed, resulting in further improvement indispersibility of the nickel powder. In addition, according to the abovepatent document, the reason for this is believed to be as follows. Whenhydroxides are present on the surfaces of nickel powder particles byadsorption or the like, since the particles are attracted to each otherdue to the hydroxyl polarity, the hydrophilic property (suspensibility)of the particles is degraded, and as a result, the nickel powderparticles aggregate with each other.

In addition, in the technique disclosed in Japanese Unexamined PatentApplication Publication No. 2003-342607, instead of water with anorganic solvent, a method has been disclosed in which after a surfactantis added to the dispersion, followed by the addition of the organicsolvent. The dispersion thus processed is held still, and the water isthen separated by decantation and is further removed by heating at 50 to150° C. In the above patent document, many types of surfactants arementioned by way of example, and according to the above disclosedtechnique, replacement of the water disperse medium with the organicsolvent can be easily performed by addition of the surfactant, andsuperior paste properties can be finally obtained. Furthermore, it hasalso been disclosed that, in general, a nonionic surfactant having anHLB (hydrophile-lipophile balance) value of 3 to 20 is preferably used.

In addition, as another technique which satisfies the aboverequirements, an ultrafine metal powder slurry having superiordispersibility was proposed (see Japanese Unexamined Patent ApplicationPublication No. 2004-158397). The ultrafine metal powder slurry is anultrafine metal powder slurry containing an organic solvent, asurfactant having a hydrophilic group and a lipophilic group, and morethan 60 to less than 95 percent by mass of an ultrafine metal powder, inwhich the hydrophilic group of the surfactant described above issulfonato group, sulfo group, sulfonyldioxyl group, polyoxyethylenegroup with carboxyl group, or polyoxyethylene group with phosphategroup, and in which the lipophilic group is an alkyl group containing 12or more carbon atoms or an alkylphenyl group.

In addition, according to Japanese Unexamined Patent ApplicationPublication No. 2003-342607, since the replacement of water with anorganic solvent is performed by physical operation using the differencein gravity and by operation removing water using evaporation, forexample, decantation operation and drying treatment are required.Accordingly, in particular, as disclosed in Example 1 of JapaneseUnexamined Patent Application Publication No. 2003-342607, dryingtreatment must be performed at 120° C. for 16 hours for 1 kg of a nickelpowder, followed by further drying treatment at 100° C. for 48 hours;hence, reduction in labor and reduction in treatment time have beenachieved in a conductive paste production process.

On the other hand, according to Japanese Unexamined Patent ApplicationPublication No. 2004-158397, an ultrafine metal powder slurry havingsuperior dispersibility can be provided; however, due to advancedquality requirements for conductive pastes, improvement in properties ofultrafine metal powder slurry itself (in particular, dispersibility, dryfilm density, and the like) has been further achieved. That is, when thedry film density is decreased, contraction of electrode films caused byfiring is increased. As a result, areas of the electrode films aredecreased or are partly broken off, and an ideal electric capacity maynot be obtained due to a decrease in the effective electrode area(covering area). The decrease in effective electrode area may also causea decrease in yield of the products. Recent technical developments ofmultilayer ceramic capacitors primarily aim at increasing the electriccapacity. In order to achieve higher electric capacity, a technique ofdecreasing the thickness of electrode films is required. When thethickness of electrode films is decreased, the number of metal particlesoverlapping each other in one layer is decreased to 4 to 8 particles,which is approximately one third of the number of particles that havebeen used in one layer. Accordingly, by particles overlapping each otherin the thickness direction of a multilayer film, an effective electrodearea after firing has been ensured; however, when the thickness ofelectrode films is decreased, it becomes difficult to obtain the aboveeffect. Accordingly, an increase in dry film density after coating, thatis, increase in particle density must be achieved.

SUMMARY

Hence, an object of the present invention is to provide an ultrafinemetal powder slurry having superior dispersibility and dry film densitythat can reduce labor and treatment time in a conductive pasteproduction process and that can prevent aggregation of the ultrafinemetal powder so that no aggregated particles are generated.

Accordingly, an exemplary embodiment of the present invention providesthe following ultrafine metal powder slurries (1) to (4).

(1) An ultrafine metal powder slurry comprising an organic solvent, asurfactant, and an ultrafine metal powder, in which the surfactant isoleoyl sarcosine, the content of the ultrafine metal powder in theultrafine metal powder slurry is in the range of 70 to 95 percent bymass, and the content of the surfactant is more than 0.05 to less than2.0 parts by mass relative to 100 parts by mass of the ultrafine metalpowder.

(2) In the ultrafine metal powder slurry described in the above (1), theparticle size distribution D90 of the ultrafine metal powder is lessthan 1.2 μm, and the particle size distribution D50, which is the meanparticle diameter, is in the range of 0.1 to 1.0 μm.

(3) In the ultrafine metal powder slurry described in the above (1) or(2), the ultrafine metal powder comprises at lease one of nickel (Ni),copper (Cu), silver (Ag), molybdenum (Mo), tungsten (W), cobalt (Co),and tantalum (Ta).

(4) In the ultrafine metal powder slurry described in the above (1) or(2), the ultrafine metal powder comprises a nickel alloy containingnickel and at least one of vanadium (V), niobium (Nb), molybdenum,tantalum, tungsten, zirconium (Zr), yttrium (Y), lanthanum (La),magnesium (Mg), titanium (Ti), barium (Ba), and calcium (Ca).

(5) An ultrafine metal powder slurry comprising about 70 to 95 percentby mass of an ultrafine metal powder, oleoyl sarcosine as a surfactantin an amount of more than 0.05 to less than 2.0 parts by mass relativeto 100 parts by mass of the ultrafine metal powder, and an organicsolvent as the balance.

In the present invention, the “particle size distribution (D90)”represents a particle diameter at a cumulative percentage of 90% (D90)on a volume basis, which is obtained in accordance with JIS R1629-1997“Determination of particle size distribution for fine ceramic rawpowders by laser diffraction method.” In addition, the “mean particlediameter D50” represents a value at a cumulative percentage of 50% (D50)on a volume basis.

As described below, according to an exemplary embodiment of the presentinvention, an ultrafine metal powder slurry having superiordispersibility and dry film density can be provided, that includes anultrafine metal powder at a significantly high content and that preventsaggregation of the ultrafine metal powder so that no aggregatedparticles are generated. In addition, this ultrafine metal powder slurrycan reduce labor and a treatment time in a conductive paste productionprocess. Furthermore, since risky operation can be avoided in whichoperators may breathe dusts generated from a dry powder, and workingenvironment can be improved, safety of workers and health environmentare significantly improved.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a flowchart showing an exemplary process for producing anultrafine metal powder slurry; and

FIG. 2 is a flowchart of a conventional process for producing anultrafine metal powder by a chemical vapor deposition method.

DETAILED DESCRIPTION OF EMBODIMENTS

In order to achieve the object described above, when an organic solventsubstitution is performed by surface chemistry reaction (neutralization)using oleoyl sarcosine (C₁₇H₃₃CON(CH₃)CH₂COOH), which is anon-neutralized acid type surfactant, without performing pH adjustmentby a carbon dioxide gas or an aqueous carbonated solution, reduction inlabor and treatment time in a conductive paste production process can beachieved. Moreover, an ultrafine metal powder slurry having superiordispersibility and dry film density can be obtained in which noaggregated particles are present.

These results are based on the knowledge that since the product can berecovered as an ultrafine metal powder slurry by the surface chemicalreaction (neutralization) using oleoyl sarcosine, decantation operationand drying operation are not required, and the knowledge that whenremoving hydroxides present on the surface of the ultrafine metalpowder, oleoyl sarcosine simultaneously adsorbs thereon so as to preventaggregation of particles of the ultrafine metal powder.

Although a general ionic surfactant is a salt type surfactant which isobtained by neutralizing an acid used as a raw material by an alkalinematerial, a “non-neutralized acid type” indicates that a non-neutralizedacid is used as a raw material.

The exemplary ultrafine metal powder slurry contains an organic solvent,a surfactant, and an ultrafine metal powder, in which the surfactant isoleoyl sarcosine, the content of the ultrafine metal powder in theultrafine metal powder slurry is 70 to 95 percent by mass, and thecontent of the surfactant relative to 100 parts by mass of the ultrafinemetal powder is more than 0.05 to less than 2.0 parts by mass. Inaddition, it may be preferable that the particle size distribution D90of the ultrafine metal powder be less than 1.2 μm, and that the particlesize distribution D50 indicating the mean particle diameter be 0.1 to1.0 μm.

An exemplary process for producing an ultrafine metal powder slurry ofthe present invention will be described below with reference to FIG. 1by way of example. However, it is naturally to be understood that theprocess for producing an ultrafine metal powder slurry is not limitedthereto.

FIG. 1 is a flowchart showing an exemplary process (Process 10) forproducing an ultrafine metal powder slurry. Purification of an ultrafinemetal powder (formation of a metal-water slurry) performed during Step11 is similar to the purification during Step 21 of Process 20 describedabove, which is a process for producing an ultrafine metal powder slurryby a chemical vapor deposition method.

In Process 10, for producing an ultrafine metal powder slurry withoutperforming a step of drying the metal powder (Step 22 shown in FIG. 2),the metal-water slurry is transferred to Step 12 of performing organicsolvent substitution in which water of the ultrafine metal powder slurryis directly replaced with an organic solvent.

In particular, for example, 0.3 parts by mass of a surfactant (oleoylsarcosine) is added relative to 100 parts by mass of the ultrafine metalpowder slurry (the content of the ultrafine metal powder being 50percent by mass), and a mixture thus formed is processed by dispersiontreatment for a predetermined period of time using a process homogenizeror the like, so that aggregates of the ultrafine metal powder in waterare dispersed into primary particles. Subsequently, as an organicsolvent, for example, 10 parts by mass of terpineol are added relativeto 100 parts by mass of the ultrafine metal powder to form a mixedsolution.

Next, the mixed solution containing terpineol thus prepared is processedby mixing treatment at a temperature of 15±5° C. for a predeterminedtime using a process homogenizer or the like. By this mixing treatment,since terpineol is adsorbed onto oleoyl sarcosine adsorbed on thesurface of the ultrafine metal powder to form a terpineol layer, andwater present around the ultrafine metal powder is thus replaced withthe terpineol.

When a terpineol layer containing the ultrafine metal powder forms acontinuous layer, the organic solvent substitution during Step 12 iscomplete, and an ultrafine metal powder-terpineol slurry composed of theultrafine metal powder, terpineol, and oleoyl sarcosine is formed into aprecipitate. The water thus replaced is immediately separated as a clearsupernatant (without being held still), and by this discharge of theclear supernatant, an ultrafine metal powder-terpineol slurry (ultrafinemetal powder-organic solvent slurry) containing 90 percent by mass ofthe ultrafine metal powder is obtained during Step 13.

Because the powder is not dried, the ultrafine metal powder-organicsolvent slurry obtained during Step 13 contains no aggregated particles,unlike the related technique described above. By adjusting the amount ofan organic solvent used during Step 12, a slurry containing 70 to 95percent by mass of the ultrafine metal powder can be obtained.

In addition, the ultrafine metal powder-organic solvent slurry asobtained during Step 13 can be used as a metal raw material for aconductive paste. Hence, during Step 14 in which viscosity adjustment isperformed, that is, in a step of forming a conductive paste, which maybe performed at a customer site, a conductive paste can be obtained byadding a binder resin (such as ethyl cellulose) solution in an amountrequired for viscosity adjustment to the above slurry. Accordingly,complicated, dispersion treatments, filtration treatment (Steps 25 and26 shown in FIG. 2), and the like can be omitted. In addition, since theultrafine metal powder-organic solvent slurry is used as a metal rawmaterial for a conductive paste instead of the dry powder as describedabove, the risks caused by dust discharge from the dry powder can beavoided, and hence the working environment can be significantlyimproved.

The ultrafine metal powder, the surfactant, and the organic solventforming the ultrafine metal powder slurry of the present invention aredescribed in detail below.

(1) Type of Metal or Metal Alloy of Ultrafine Metal Powder

The exemplary ultrafine metal powder is not particularly limited to anyspecific metal, as long as it is a metal or an alloy having a meanparticle diameter D50 of 0.1 to 1.0 μm, and particles of the metalpowder preferably have an approximately spherical shape. In thisexemplary embodiment, “approximately spherical shape” is defined suchthat the ratio of the maximum length to the minimum width of a particle,that is, the value (aspect ratio=maximum length/minimums width) obtainedby dividing the maximum length by the minimum width is in the range of 1to less than 1.8. For measurement of the aspect ratio, the aspect ratiosof 500 samples observed by a scanning electron microscope are measuredand are then averaged, thereby obtaining the aspect ratio.

As the type of metal or alloy described above, in particular, forexample, nickel, copper, silver, molybdenum, tungsten, cobalt, andtantalum may be used alone or in combination. Among those mentionedabove, nickel, copper, silver, and tantalum is preferably used becausesuperior electrical conduction can be obtained.

In particular, for forming a conductive paste, a nickel alloy ispreferably used that contains nickel and at least one metal elementselected from vanadium, niobium, molybdenum, tantalum, tungsten,zirconium, yttrium, lanthanum, magnesium, titanium, barium, and calcium,in an amount of 0.03 to 10 parts by mass relative to 100 parts by massof nickel particle. When a paste is formed using the nickel alloydescribed above, the heat contraction of the paste is preferably smallafter application thereof.

The exemplary ultrafine metal powder composed of the metal or the alloydescribed above can be formed by a known method such as a gas phasemethod or a liquid phase method. In particular, production is preferablyperformed by chemical vapor deposition in which, after a metal chlorideis evaporated, a metal powder is obtained by reduction using H₂ gas.When the production is performed using chemical vapor deposition, theparticle diameter of the metal powder thus produced can be easilycontrolled, and in addition, spherical particles can be efficientlyproduced.

(2) Content of Ultrafine Metal Powder: 70 to 95 Percent by Mass

The content of the exemplary ultrafine metal powder in the ultrafinemetal powder slurry may be 70 to 95 percent by mass. The reason thecontent of the ultrafine metal powder can be significantly increased asdescribed above is that a film of an organic solvent forms on thesurface of each particle of the ultrafine metal powder. A slurrycomposed of an ultrafine metal powder, an organic solvent, and asurfactant is thus formed as an intermediate product in a pasteproduction process. However, the content of the ultrafine metal powdermay be approximately 50 percent by mass or less. In order to increasethe dry film density, a slurry containing a larger amount of anultrafine metal powder is preferably used. However, when a large amountthereof is contained in a slurry, even if complicated dispersiontreatment is performed, it is generally difficult to disaggregateaggregates of the ultrafine metal powder in the slurry and to ensuredispersibility. When production of ceramic capacitors is performed usinga slurry containing aggregates as described above as a raw metalmaterial for conductive paste, it is difficult to obtain an appropriateperformance of ceramic capacitors.

A metal slurry containing 70 to 95 percent by mass of an ultrafine metalpowder and having superior dispersibility and a higher dry film densityis generally difficult to obtain. However, the metal slurry describedabove can be realized. Without forming aggregates, substantially primaryparticles of an exemplary ultrafine metal powder are dispersed in anorganic solvent so as to form a dense and uniform matrix.

When the content of the exemplary ultrafine metal powder is less than 70percent by mass, since some parts between metal particles contain alarger amount of the organic solvent to form a non-uniform matrix, thedry film density is decreased. The adjustment of the content of theultrafine metal powder is performed by adjusting the amount of theorganic solvent. When the content of the ultrafine metal powder is morethan 95 percent by mass, the amount of the organic solvent added theretois insufficient and is adsorbed onto the peripheries of the particles ofthe ultrafine metal powder. As a result, organic solvent layer cannot beformed due to the insufficient amount thereof, so that aggregates of theultrafine metal powder are locally formed. As a result, a non-uniformmatrix is also formed in this case, and the dry film density decreases.The content of the exemplary ultrafine metal powder is more preferablyin the range of more than 80 to less than 93 percent by mass.

In addition, the adjustment of the content of the ultrafine metal powdercan be performed by the adjustment of the amount of an organic solventas described below. However, when the content of the ultrafine metalpowder is in the range of 70 to 95 percent by mass, the dry film densityof a conductive paste obtained from the ultrafine metal powder slurry ofthe present invention can be increased, and in addition, the amount ofan organic solvent is sufficient to form an organic solvent layer,thereby suppressing the formation of aggregates of the ultrafine metalpowder.

(3) Particle Size Distribution D90 of Ultrafine Metal Powder of Lessthan 1.2 μm, and Mean Particle Diameter D50 of 0.1 to 1.0 μm

In addition, the particle size distribution D90 of the exemplaryultrafine metal powder is preferably less than 1.2 μm and morepreferably less than 1.0 μm.

As described above, of the particle size distribution of an ultrafinemetal powder in an organic solvent slurry which is measured using alaser particle size analyzer, the particle size distribution D90 is aparticle diameter at a cumulative percentage of 90% on a volume basis,and a laser particle size analyzer is generally used for measuring thedispersion state of metal particles dispersed in an organic solvent.

When the particle size distribution D90 of the ultrafine metal powder isless than 1.2 μm, since the dispersibility is satisfactory, and denseand smooth electrode layers are formed, a superior ceramic capacitor canbe obtained. In order to decrease the number of projections of theelectrode layers, D90 is more preferably less than 1.0 μm.

In addition, the mean particle diameter D50 of the ultrafine metalpowder is preferably 0.1 to 1.0 μm. As for the mean particle diameterD50, in order to decrease D90, particles having a small D50 arepreferably used. When metal particles having a mean particle diameterD50 of less than 1.0 μm are used, D90 can be decreased to less than 1.2μm. Furthermore, in view of the number of projections, particles havinga mean particle diameter D50 of 0.61 μm or less are preferably used.Hence, the mean particle diameter D50 is preferably 0.61 μm or less.Particles having a mean particle diameter D50 of less than 0.1 μm havehigh surface activity and are not practically used. However, when thesurface activity of the particles can be decreased. The particles may beused to achieved the desired results discussed above.

The “particle size distribution (D90)” represents a particle diameter ata cumulative percentage of 90% (D90) on a volume basis, which isobtained in accordance with JIS R1629-1997 “Determination of particlesize distribution for fine ceramic raw powders by laser diffractionmethod”. In addition, the “mean particle diameter D50” represents avalue at a cumulative percentage of 50% (D50) on a volume basis.

(4) Surfactant: Oleoyl Sarcosine, Non-Neutralized Acid Type Surfactant

A surfactant used is oleoyl sarcosine which is a non-neutralized acidtype surfactant.

The properties required for a non-neutralized acid type surfactant andthe reason oleoyl sarcosine is used will be described with reference tothe case of an ultrafine nickel powder by way of example. In the casesof other ultrafine metal powders other than Ni, the following mechanismcan also be applied thereto.

On the surface of the ultrafine Ni powder dispersed in water, there areparts on which oxide layers are present and parts on which metal Ni ispresent. In addition, on the surface of the ultrafine Ni powder, it hasbeen believed that two types of hydrophilic OH groups are present due totwo different formation mechanisms.

(1) OH groups formed by adsorption of water molecules on the surface ofthe ultrafine Ni powder, followed by releasing protons (H⁺)(deprotonation))

(2) OH groups derived from Ni hydroxides formed by reaction betweenwater molecules and metal ions ionized on the surface of the ultrafineNi powder

Hereinafter, the formation mechanism of the above Ni hydroxides will bedescribed in detail. Ni ions eluted from the surface of the ultrafine Nipowder are formed into aquo-ions having water molecules as a ligand.Next, by deprotonation, hydroxyl ions are generated. Polynuclear complexions are formed when adjacent Ni ions are bonded together with hydroxylions provided therebetween. The polynuclear complex ions aredeprotonated and are then bonded to adjacent polynuclear complex ions,and hence the Ni hydroxides are formed on the surface of the ultrafineNi powder. In the molecular geometry of this Ni hydroxide, a hydrophilicOH group and a water molecule are present as a ligand. It is believedthat by the effect of this hydrophilic OH group and the presence of thewater molecule, the affinity for water used as a disperse medium isimproved. That is, it has been believed that on the surface of theultrafine Ni powder dispersed in water, structures having superioraffinity for water, that is, hydrophilic OH groups are present.

Hence, in order to replace water in the metal-water slurry with anorganic solvent which is insoluble in water, the OH groups of the above(1) and (2) which are present on the surface of the ultrafine Ni powdermust be removed.

Accordingly, the surfactant is required to have a function of removingOH groups from the surface of the ultrafine Ni powder by neutralizingthe OH groups present thereon with protons (H⁺) so that Ni hydroxidesare dissolved in water. In addition, the surfactant is also required tobe adsorbed on the surfaces of particles of the ultrafine Ni powder fromwhich the OH groups are removed so as to function as a steric hindranceto prevent aggregation of the ultrafine Ni powder.

After the organic solvent substitution is performed, when a plurality ofultrafine metal particles having hydroxides on the surfaces thereof ispresent, since having no affinity for an organic solvent, the aboveultrafine Ni particles aggregate with each other by a water medium(water remaining after replacement performed with an organic solvent),and as a result, the dispersibility of the slurry is degraded.

The present invention was made on the new knowledge in which as asurfactant satisfying the functions described above, a non-neutralizedacid type surfactant must be used. For example, by the surfactantsdisclosed in Japanese Unexamined Patent Application Publication Nos.2003-342607 and 2004-158397, which were previously discussed in RelatedArt, since the OH groups on the surface of the ultrafine Ni particlesare not totally neutralized, and a considerable amount of water stillremains after replacement performed with the organic solvent, aggregatesare formed, and as a result, the dispersibility of the slurry isdegraded.

In addition, the present invention was made on the new knowledge inwhich by particularly using oleoyl sarcosine among variousnon-neutralized acid type surfactants, in addition to the improvement indispersibility of the ultrafine metal powder slurry, the dry filmdensity can also be improved.

Besides the improvement in dispersibility of the ultrafine metal powderslurry, the reasons the dry film density can also be improved byparticularly using oleoyl sarcosine among various non-neutralized acidtype surfactants have not been clearly understood; however, the abovephenomenon has been construed as follows.

Table 1 shows experimental data of viscosities of metal slurriesprepared by using various types of non-neutralized acid typesurfactants. In Experimental Example 1 in which oleoyl sarcosine wasused, the viscosity of the Ni metal slurry was significantly decreasedas compared to that of the other non-neutralized acid type surfactants(Experimental Examples 2 to 4). Since a low viscosity of a highconcentration slurry is used as an index indicating superiordispersibility, the slurry obtained in Experimental Example 1 in whicholeoyl sarcosine was used had improved dispersibility as compared tothat obtained in Experimental Examples 2, 3, and 4 in which the othernon-neutralized acid type surfactants were used. The reason for this isthat oleoyl sarcosine is adsorbed onto the Ni metal particles so as todecrease a friction force between the metal particles. By the functiondescribed above, it is believed that metal particles are easily moved informing a dry film, and as a result, the Ni metal particles are easilyand densely compacted. The reason for this is that the adsorption modeof oleoyl sarcosine and that of the other surfactants are different fromeach other due to the difference in chemical difference in chemicalstructure therebetween. When oleoyl sarcosine of experimental example 1is used, it is also believed that since the carbonyl group and theunpaired electron of the nitrogen atom effectively function as anadsorption site, the functional group, i.e., the C₁₇H₃₃ alkyl groupadsorbs an organic solvent so that a uniform organic solvent film isformed on the surface of the Ni metal powder. When the slurry describedabove is used for a conductive paste, since ethyl cellulose added as abinder is cross-linked with the carbonyl group and the unpaired electronof the nitrogen atom of oleoyl sarcosine, and hence ethyl cellulose canalso be easily dispersed around the Ni metal particles. That is, it isbelieved that the surfactant described above also has the function ofpreventing the formation of aggregates of the binder. By the effect ofoleoyl sarcosine described above, the dry film density of a conductivepaste is believed to be improved.

Oleoyl sarcosine used in the present invention can performneutralization of OH groups by protons, which are present on the surfaceof an ultrafine metal powder in a metal-water slurry, simultaneous withchemical adsorption reaction on the surface of the ultrafine metalpowder, and as a result, a monomolecular film adsorbed on the entiresurface of the ultrafine metal powder is formed.

In this monomolecular film thus adsorbed, the lipophilic group (C₁₇H₃₃—)of oleoyl sarcosine is located outside the molecule. Accordingly, whenan organic solvent molecule is adsorbed on this lipophilic group, anorganic solvent layer is formed around each particle of the ultrafinemetal powder. When a plurality of particles of the ultrafine metalpowder wrapped in the organic solvent layers gather together and exceeda critical point at which a continuous film can be sufficiently formedwith the organic solvent layers, an ultrafine metal powder slurry of theorganic solvent (ultrafine metal powder-organic solvent slurry) isformed.

Since having a specific density of 5 to 6 g/cm³, which is much largerthan that of water, this ultrafine metal powder-organic solvent slurryprecipitates and is then recovered as a reaction product.

In addition, water molecules adsorbed on the ultrafine metal powder areremoved when the organic solvent molecules are adsorbed thereon, and thecontinuous layer is formed around the ultrafine metal powder by theorganic solvent. The ability of removing water molecules effectivelyworks when a lipophilic group of a surfactant has low affinity for awater molecule and has high affinity for an organic solvent molecule.Hence, oleoyl sarcosine, which has a (C₁₇H₃₃—) group as a lipophilicgroup, is very useful since having high ability of removing watermolecules and being capable of effectively removing water moleculesadsorbed on an ultrafine metal powder.

As described above, since having a lipophilic group, oleoyl sarcosinehas the function of forming an organic solvent layer around an ultrafinemetal powder. By the function described above, since particles of theultrafine metal powder are each wrapped in an organic solvent, organicsolvent substitution can be performed in which an ultrafine metal powderin a metal-water slurry can be transferred in an organic solvent. Inaddition, since having affinity for a different organic solvent which isadditionally used when a paste is formed, oleoyl sarcosine can uniformlydistribute an ultrafine metal powder in the paste, and as a result, thedry film density of a conductive paste can be increased.

In a conductive paste formed in accordance with Japanese UnexaminedPatent Application Publication No. 2004-158397, the dry film densitycould be increased only up to 5.6 g/cm³ when an ultrafine Ni powderhaving a mean particle diameter of 0.4 μm was used; however, inparticular, as also shown in examples described later, in a conductivepaste formed from a slurry containing oleoyl sarcosine as a surfactantin accordance with the present invention, the dry film density can beincreased to 5.8 g/cm³ or more.

In addition to the effect of a lipophilic (C₁₇H₃₃—) group dispersing anultrafine metal powder in an organic solvent, the reason the dense filmis formed as described above is that a carbonyl group (C═O) and anunpaired electron are bonded with ethyl cellulose used in forming aconductive paste by acid-base reaction.

(5) Content of Oleoyl Sarcosine: More than 0.05 to Less than 2.0 Partsby Mass Relative to 100 Parts by Mass of the Ultrafine Metal Powder

The content of the surfactant (oleoyl sarcosine) which is uniformlyadsorbed on the entire surface of an ultrafine metal powder in ametal-water slurry is an appropriate amount, and the content of oleoylsarcosine is in the range of more than 0.05 to less than 2.0 parts bymass relative to 100 parts by mass of the ultrafine metal powder. Whenthe content of oleoyl sarcosine is 0.05 parts by mass or less relativeto 100 parts by mass of the ultrafine metal powder, oleoyl sarcosine isnot sufficiently adsorbed on the entire surface of the ultrafine metalpowder, and the organic solvent substitution cannot be satisfactorilyperformed; hence, the content is preferably more than 0.05 parts bymass. On the other hand, when the content of oleoyl sarcosine is 2.0parts by mass or more relative to 100 parts by mass of the ultrafinemetal powder, since the content of oleoyl sarcosine exceeds the amountwhich is uniformly adsorbed on the entire surface of the ultrafine metalpowder, the excessive amount of oleoyl sarcosine has little effect andis not economical; hence, the content is preferably set to less than 2.0parts by mass. When the content of oleoyl sarcosine is in the range ofmore than 0.05 to less than 2.0 parts by mass, the surfactant issufficiently adsorbed on the entire surface of the ultrafine metalpowder, and a monomolecular layer adsorbed thereon can be formed.Accordingly, preferably, the organic solvent substitution can be easilyperformed, the dry film density obtained from a conductive paste isincreased, and economical advantages can also be obtained.

(6) Organic Solvent

The content of the organic solvent used in the present invention is thebalance obtained by deducting the total of the ultrafine metal powderand the surfactant from the ultrafine metal powder slurry, which aredefined in the present invention. Although any organic solvent may beused as long as it is generally used as a solvent for a conductivepaste, for example, terpene alcohol solvents and aliphatic hydrocarbonsolvents may be preferably mentioned. As the terpene alcohols, forexample, terpineol, dihydroterpineol, terpineol acetate, borneol,geraniol, and linalool may be mentioned and may be used alone or incombination.

As the aliphatic hydrocarbon alcohols, n-decane, n-dodecane, and mineralspirit may be mentioned by way of example and may be used alone or incombination.

In the present invention, since being determined by the content of theultrafine metal powder and the content of oleoyl sarcosine describedabove, the content of the organic solvent described above isapproximately 3.1 to 30 percent by mass (approximately 3.2 to 42 partsby mass relative to 100 parts by mass of the ultrafine metal powder).

The ultrafine metal powder slurry of the present invention can bepreferably used as a raw material for a conductive paste due to thevarious properties described above and can be used as conductive pastefillers or internal electrodes of multilayer ceramic capacitors.

EXAMPLES

Hereinafter, the present invention will be described with reference toexamples; however, it is to be naturally understood that the presentinvention is not limited thereto.

Example 1

First, 10 liters of an ultrafine Ni powder-water slurry (content ofultrafine Ni powder of 50 percent by mass) were prepared, the ultrafineNi powder having a high purity and a mean particle diameter of 0.4 μm.This slurry corresponds to the slurry obtained by the purification ofultrafine metal powder during Step 11 shown in FIG. 1.

Next, 0.3 parts by mass of oleoyl sarcosine (Sarcosinate OH manufacturedby Nikko Chemical Co., Ltd.) used as a surfactant was added relative to100 parts by mass of this ultrafine Ni powder slurry. Subsequently, at atemperature of 15° C.±5° C., pretreatment was performed at a bladerotation speed of 800 rotations per minute (rpm) for 30 minutes by adispersion device using a process homogenizer (manufactured by SMT Co.,Ltd).

Next, to the ultrafine Ni powder-water slurry thus pretreated, 10 partsby mass of terpineol (manufactured by Yasuhara Chemical Co., Ltd.) usedas an organic solvent relative to 100 parts by mass of the ultrafine Nipowder was added to form a mixture. The mixture thus formed wasprocessed at a temperature of 15° C.±5° C. by a dispersion device usinga process homogenizer (manufactured by SMT Co., Ltd.) at a bladerotation speed of 5,000 rpm for 15 minutes. Accordingly, water presentaround the ultrafine Ni powder was replaced with terpineol, and as aresult, an ultrafine Ni powder-terpineol slurry was obtained as aprecipitate in water.

Subsequently, a separated clear supernatant was discharged, and anultrafine Ni powder-terpineol slurry (Ni-organic solvent slurry)containing 90 percent by mass of the ultrafine Ni powder was obtained,the slurry being composed of the ultrafine Ni powder, terpineol, andoleoyl sarcosine.

Solvent Substitution

Solvent substitution of the ultrafine Ni powder-terpineol slurry thusformed was evaluated based on the following criteria. That is,substitution totally performed was evaluated as “Good” by ◯,substitution partly performed (powdered Ni floating in a supernatant)was evaluated as “Fair” by Δ, and substitution insufficiently performed(no formation of ultrafine Ni powder-terpineol slurry) was evaluated as“Poor” by x. The results are shown in Table 2 below.

In addition, the water content of the ultrafine Ni powder-organicsolvent slurry after the solvent substitution was measured using a KarlFisher moisture meter. When the amount of remaining water is smaller, itindicates that more superior solvent substitution is obtained.Furthermore, it also indicates that the generation of Ni powderaggregates caused by water remaining in the organic solvent issignificantly suppressed. The results are shown in Table 2 below.

Measurement of Particle Size Distribution

The particle size distribution of the ultrafine Ni powder-terpineolslurry thus obtained was measured using a laser particle size analyzerunder the following conditions. In this measurement, after a solutionprocessed by pre-dispersion treatment was charged into the analyzeruntil a predetermined absorbance was obtained, the measurement wasperformed.

Measurement device: Laser particle size analyzer (SALD-2100 manufacturedby Shimadzu Corporation)

Refractive index: 1.60

Sample mass: 30.00 to 36.00 mg

Dispersion solution: 100 ml of terpineol

Pre-dispersion treatment: ultrasonic homogenizer (US-600 manufactured byNippon Seiki Seisakusho Co., Ltd).

Pre-dispersion time: 5 minutes

The dispersibility was evaluated by the particle size distribution D90based on the following criteria. That is, a D90 of less than 1.2 μm wasevaluated as “Good” by ◯, a D90 of 1.2 μm to less than 1.5 μm wasevaluated as “Fair” by Δ, and a D90 of 1.5 μm to less than 2.0 μm wasevaluated as “Poor” by x. The results are shown in Table 2 below.

Measurement of Dry Film Density (ρG: g/cm³)

To 111 parts by mass of the obtained ultrafine Ni powder-terpineolslurry (content of the ultrafine Ni powder of 90 percent by mass), 62.5parts by mass of a binder resin solution was added in which 8 percent bymass of ethyl cellulose was contained in terpineol, and after themixture thus formed was mixed by an agitator for 30 minutes, terpineolwas added thereto for viscosity adjustment so that the content of theultrafine Ni powder was approximately 50 percent by mass, therebyforming a conductive paste.

The conductive paste thus prepared was applied by an applicator onto aPET (polyethylene terephthalate) film processed beforehand by releasetreatment so as to obtain a smooth coating surface. After the PET filmprovided with the conductive paste by application was dried by a hotplate which was set at a temperature of 80 to 150° C., the conductivepaste was peeled away from the PET film, thereby forming a dry film. Byusing a cylindrical punching tool, a circular film having a diameter of2 cm was obtained from the dry film thus formed.

From the mass and the volume of the circular film thus obtained, the dryfilm density was calculated. As for the volume, the thickness wasmeasured at several points (approximately 5 to 6 points) using amicrometer, and the average thereof was obtained therefrom. The resultsare shown in Table 2 below. A dry film having a density of 5.8 g/cm³ ormore was regarded as a dense film in the present invention.

Example 2

An ultrafine Ni powder-terpineol slurry was obtained in the same manneras that in Example 1 except that 0.1 parts by mass of oleoyl sarcosine(Sarcosinate OH manufactured by Nikko Chemical Co., Ltd.) was addedrelative to 100 parts by mass of the ultrafine Ni powder.

Example 3

An ultrafine Ni powder-terpineol slurry was obtained in the same manneras that in Example 1 except that 1.0 part by mass of oleoyl sarcosine(Sarcosinate OH manufactured by Nikko Chemical Co., Ltd.) was addedrelative to 100 parts by mass of the ultrafine Ni powder.

Example 4

An ultrafine Ni powder-terpineol slurry was obtained in the same manneras that in Example 1 except that 2.0 parts by mass of oleoyl sarcosine(Sarcosinate OH manufactured by Nikko Chemical Co., Ltd.) was addedrelative to 100 parts by mass of the ultrafine Ni powder.

Example 5

An ultrafine Ni powder-terpineol slurry (content of the ultrafine Nipowder of 80 percent by mass) was obtained in the same manner as that inExample 1 except that 25 parts by mass of terpineol was added relativeto 100 parts by mass of the ultrafine Ni powder.

Example 6

An ultrafine Ni powder-terpineol slurry (content of the ultrafine Nipowder of 95 percent by mass) was obtained in the same manner as that inExample 1 except that 5.3 parts by mass of terpineol was added relativeto 100 parts by mass of the ultrafine Ni powder.

Example 7

An ultrafine Ni powder-terpineol slurry (content of the ultrafine Nipowder of 70 percent by mass) was obtained in the same manner as that inExample 1 except that 42 parts by mass of terpineol was added relativeto 100 parts by mass of the ultrafine Ni powder.

Example 8

An ultrafine Ni powder-dodecane slurry was obtained in the same manneras that in Example 1 except that n-dodecane (n-C₁₂H₂₆), an aliphatichydrocarbon, was used as the organic solvent.

Example 9

An ultrafine Ni powder-dihydroterpineol slurry was obtained in the samemanner as that in Example 1 except that dihydroterpineol was used as theorganic solvent.

Example 10

An ultrafine Ni powder-terpineol acetate slurry was obtained in the samemanner as that in Example 1 except that terpineol acetate was used asthe organic solvent.

Example 11

An ultrafine Cu powder-terpineol slurry was obtained in the same manneras that in Example 1 except that an ultrafine Cu powder having a meanparticle diameter of 0.4 μm was used as the ultrafine metal powder, andthat 0.3 parts by mass of oleoyl sarcosine (Sarcosinate OH manufacturedby Nikko Chemical Co., Ltd.) was added relative to 100 parts by mass ofthe ultrafine Cu powder.

Example 12

An ultrafine Ag powder-terpineol slurry was obtained in the same manneras that in Example 1 except that an ultrafine Ag powder having a meanparticle diameter of 0.4 μm was used as the ultrafine metal powder, andthat 0.3 parts by mass of oleoyl sarcosine (Sarcosinate OH manufacturedby Nikko Chemical Co., Ltd.) was added relative to 100 parts by mass ofthe ultrafine Ag powder.

Example 13

An ultrafine Mo powder-terpineol slurry was obtained in the same manneras that in Example 1 except that an ultrafine Mo powder having a meanparticle diameter of 0.4 μm was used as the ultrafine metal powder, andthat 0.3 parts by mass of oleoyl sarcosine (Sarcosinate OH manufacturedby Nikko Chemical Co., Ltd.) was added relative to 100 parts by mass ofthe ultrafine Mo powder.

Example 14

An ultrafine W powder-terpineol slurry was obtained in the same manneras that in Example 1 except that an ultrafine W powder having a meanparticle diameter of 0.4 μm was used as the ultrafine metal powder, andthat 0.3 parts by mass of oleoyl sarcosine (Sarcosinate OH manufacturedby Nikko Chemical Co., Ltd.) was added relative to 100 parts by mass ofthe ultrafine W powder.

Example 15

An ultrafine Co powder-terpineol slurry was obtained in the same manneras that in Example 1 except that an ultrafine Co powder having a meanparticle diameter of 0.4 μm was used as the ultrafine metal powder, andthat 0.3 parts by mass of oleoyl sarcosine (Sarcosinate OH manufacturedby Nikko Chemical Co., Ltd.) was added relative to 100 parts by mass ofthe ultrafine Co powder.

Example 16

An ultrafine Ta powder-terpineol slurry was obtained in the same manneras that in Example 1 except that an ultrafine Ta powder having a meanparticle diameter of 0.4 μm was used as the ultrafine metal powder, andthat 0.3 parts by mass of oleoyl sarcosine (Sarcosinate OH manufacturedby Nikko Chemical Co., Ltd.) was added relative to 100 parts by mass ofthe ultrafine Ta powder.

Example 17

An ultrafine Ni—V alloy powder-terpineol slurry was obtained in the samemanner as that in Example 1 except that a nickel-vanadium alloy powder(Ni:V=95:5) having a mean particle diameter of 0.4 μm was used as theultrafine metal powder, and that 0.3 parts by mass of oleoyl sarcosine(Sarcosinate OH manufactured by Nikko Chemical Co., Ltd.) was addedrelative to 100 parts by mass of the ultrafine nickel-vanadium alloypowder.

Example 18

An ultrafine Ni—Cr alloy powder-terpineol slurry was obtained in thesame manner as that in Example 1 except that a nickel-chromium alloypowder (Ni:Cr=95:5) having a mean particle diameter of 0.4 μm was usedas the ultrafine metal powder, and that 0.3 parts by mass of oleoylsarcosine (Sarcosinate OH manufactured by Nikko Chemical Co., Ltd.) wasadded relative to 100 parts by mass of the ultrafine nickel-chromiumalloy powder.

Example 19

An ultrafine Ni—Nb alloy powder-terpineol slurry was obtained in thesame manner as that in Example 1 except that a nickel-niobium alloypowder (Ni:Nb=95:5) having a mean particle diameter of 0.4 μm was usedas the ultrafine metal powder, and that 0.3 parts by mass of oleoylsarcosine (Sarcosinate OH manufactured by Nikko Chemical Co., Ltd.) wasadded relative to 100 parts by mass of the ultrafine nickel-niobiumalloy powder.

Example 20

An ultrafine Ni—Mo alloy powder-terpineol slurry was obtained in thesame manner as that in Example 1 except that a nickel-molybdenum alloypowder (Ni:Mo=95:5) having a mean particle diameter of 0.4 μm was usedas the ultrafine metal powder, and that 0.3 parts by mass of oleoylsarcosine (Sarcosinate OH manufactured by Nikko Chemical Co., Ltd.) wasadded relative to 100 parts by mass of the ultrafine nickel-molybdenumalloy powder.

Example 21

An ultrafine Ni—Ta alloy powder-terpineol slurry was obtained in thesame manner as that in Example 1 except that a nickel-tantalum alloypowder (Ni:Ta=95:5) having a mean particle diameter of 0.4 μm was usedas the ultrafine metal powder, and that 0.3 parts by mass of oleoylsarcosine (Sarcosinate OH manufactured by Nikko Chemical Co., Ltd.) wasadded relative to 100 parts by mass of the ultrafine nickel-tantalumalloy powder.

Example 22

An ultrafine Ni—W alloy powder-terpineol slurry was obtained in the samemanner as that in Example 1 except that a nickel-tungsten alloy powder(Ni:W=95:5) having a mean particle diameter of 0.4 μm was used as theultrafine metal powder, and that 0.3 parts by mass of oleoyl sarcosine(Sarcosinate OH manufactured by Nikko Chemical Co., Ltd.) was addedrelative to 100 parts by mass of the ultrafine nickel-tungsten alloypowder.

Example 23

An ultrafine Ni—Zr alloy powder-terpineol slurry was obtained in thesame manner as that in Example 1 except that a nickel-zirconium alloypowder (Ni:Zr=95:5) having a mean particle diameter of 0.4 μm was usedas the ultrafine metal powder, and that 0.3 parts by mass of oleoylsarcosine (Sarcosinate OH manufactured by Nikko Chemical Co., Ltd.) wasadded relative to 100 parts by mass of the ultrafine nickel-zirconiumalloy powder.

Example 24

An ultrafine Ni—Y alloy powder-terpineol slurry was obtained in the samemanner as that in Example 1 except that a nickel-yttrium alloy powder(Ni:Y=95:5) having a mean particle diameter of 0.4 μm was used as theultrafine metal powder, and that 0.3 parts by mass of oleoyl sarcosine(Sarcosinate OH manufactured by Nikko Chemical Co., Ltd.) was addedrelative to 100 parts by mass of the ultrafine nickel-yttrium alloypowder.

Example 25

An ultrafine Ni—La alloy powder-terpineol slurry was obtained in thesame manner as that in Example 1 except that a nickel-lanthanum alloypowder (Ni:La=95:5) having a mean particle diameter of 0.4 μm was usedas the ultrafine metal powder, and that 0.3 parts by mass of oleoylsarcosine (Sarcosinate OH manufactured by Nikko Chemical Co., Ltd.) wasadded relative to 100 parts by mass of the ultrafine nickel-lanthanumalloy powder.

Example 26

An ultrafine Ni—Mg alloy powder-terpineol slurry was obtained in thesame manner as that in Example 1 except that a nickel-magnesium alloypowder (Ni:Mg=95:5) having a mean particle diameter of 0.4 μm was usedas the ultrafine metal powder, and that 0.3 parts by mass of oleoylsarcosine (Sarcosinate OH manufactured by Nikko Chemical Co., Ltd.) wasadded relative to 100 parts by mass of the ultrafine nickel-magnesiumalloy powder.

Example 27

An ultrafine Ni—Ti alloy powder-terpineol slurry was obtained in thesame manner as that in Example 1 except that a nickel-titanium alloypowder (Ni:Ti=95:5) having a mean particle diameter of 0.4 μm was usedas the ultrafine metal powder, and that 0.3 parts by mass of oleoylsarcosine (Sarcosinate OH manufactured by Nikko Chemical Co., Ltd.) wasadded relative to 100 parts by mass of the ultrafine nickel-titaniumalloy powder.

Example 28

An ultrafine Ni—Ba alloy powder-terpineol slurry was obtained in thesame manner as that in Example 1 except that a nickel-barium alloypowder (Ni:Ba=95:5) having a mean particle diameter of 0.4 μm was usedas the ultrafine metal powder, and that 0.3 parts by mass of oleoylsarcosine (Sarcosinate OH manufactured by Nikko Chemical Co., Ltd.) wasadded relative to 100 parts by mass of the ultrafine nickel-barium alloypowder.

Example 29

An ultrafine Ni—Ca alloy powder-terpineol slurry was obtained in thesame manner as that in Example 1 except that a nickel-calcium alloypowder (Ni:Ca=95:5) having a mean particle diameter of 0.4 μm was usedas the ultrafine metal powder, and that 0.3 parts by mass of oleoylsarcosine (Sarcosinate OH manufactured by Nikko Chemical Co., Ltd.) wasadded relative to 100 parts by mass of the ultrafine nickel-calciumalloy powder.

Example 30

An ultrafine Ni—W—Ca alloy powder-terpineol slurry was obtained in thesame manner as that in Example 1 except that a nickel-tungsten-calciumalloy powder (Ni:W:Ca=95:3:2) having a mean particle diameter of 0.4 μmwas used as the ultrafine metal powder, and that 0.3 parts by mass ofoleoyl sarcosine (Sarcosinate OH manufactured by Nikko Chemical Co.,Ltd.) was added relative to 100 parts by mass of the ultrafinenickel-tungsten-calcium alloy powder.

Example 31

An ultrafine Ni—Mg—Zr alloy powder-terpineol slurry was obtained in thesame manner as that in Example 1 except that anickel-magnesium-zirconium alloy powder (Ni:Mg:Zr=95:3:2) having a meanparticle diameter of 0.4 μm was used as the ultrafine metal powder, andthat 0.3 parts by mass of oleoyl sarcosine (Sarcosinate OH manufacturedby Nikko Chemical Co., Ltd.) was added relative to 100 parts by mass ofthe ultrafine nickel-magnesium-zirconium alloy powder.

Example 32

An ultrafine Ni—Mo—Mn alloy powder-terpineol slurry was obtained in thesame manner as that in Example 1 except that anickel-molybdenum-manganese alloy powder (Ni:Mo:Mn=95:3:2) having a meanparticle diameter of 0.4 μm was used as the ultrafine metal powder, andthat 0.3 parts by mass of oleoyl sarcosine (Sarcosinate OH manufacturedby Nikko Chemical Co., Ltd.) was added relative to 100 parts by mass ofthe ultrafine nickel-molybdenum-manganese alloy powder.

Example 35

A conductive paste was obtained in the same manner as that in Example 1of the present invention except that 0.06 parts by mass of oleoylsarcosine was used as the surfactant.

Comparative Example 1

An ultrafine Ni powder-terpineol slurry was formed through the samesteps as Steps 21, 22, 23, and 24 of Process 20 in FIG. 2 for forming anintermediate paste by the related chemical vapor deposition (CVD)method.

In particular, to 1,000 g of a highly pure ultrafine Ni powder (drypowder) formed by a CVD method having a mean particle diameter of 0.4μm, terpineol (manufactured by Yasuhara Chemical Co., Ltd.) was added ata ratio of 1 to 1 on a mass basis, so that content of the ultrafine Nipowder was set to 50 percent by mass.

Next, 0.5 parts by mass of oleoyl sarcosine (Sarcosinate OH manufacturedby Nikko Chemical Co., Ltd.) was added relative to 100 parts by mass ofthe ultrafine Ni powder. Subsequently, dispersion treatment wasperformed for 1 hour by a cake mixer, thereby forming an ultrafine Nipowder-terpineol slurry.

Comparative Example 2

An ultrafine Ni powder-terpineol slurry (content of ultrafine Ni powderof 97 percent by mass) was obtained in the same manner as that inComparative Example 1 except that 3 parts by mass of terpineol(manufactured by Yasuhara Chemical Co., Ltd.) was added relative to 100parts by mass of the ultrafine Ni powder.

Comparative Example 3

An ultrafine Ni powder-terpineol slurry (content of ultrafine Ni powderof 90 percent by mass) was obtained in the same manner as that inComparative Example 1 except that 0.01 parts by mass of oleoyl sarcosine(Sarcosinate OH manufactured by Nikko Chemical Co., Ltd.) was addedrelative to 100 parts by mass of the ultrafine Ni powder.

Comparative Example 4

An ultrafine Ni powder-terpineol slurry (content of ultrafine Ni powderof 90 percent by mass) was obtained in the same manner as that inComparative Example 1 except that 0.04 parts by mass of oleoyl sarcosine(Sarcosinate OH manufactured by Nikko Chemical Co., Ltd.) was addedrelative to 100 parts by mass of the ultrafine Ni powder.

Comparative Example 5

An ultrafine Ni powder-terpineol slurry was obtained in the same manneras that in Example 1 except that carboxylated polyoxyethylene alkylether (ECT-7 manufactured by Nikko Chemical Co., Ltd.) was used as thesurfactant.

Comparative Example 6

An ultrafine Ni powder-terpineol slurry was obtained in the same manneras that in Example 1 except that polyoxyethylene lauryl ether acetate(RLM-45 manufactured by Nikko Chemical Co., Ltd.) was used as thesurfactant.

Comparative Example 7

An ultrafine Ni powder-terpineol slurry was obtained in the same manneras that in Example 1 except that alkylbenzenesulfonic acid (Lipon LH-200manufactured by Lion Corporation) was used as the surfactant, and that0.2 parts by mass thereof was added relative to 100 parts by mass of theultrafine Ni powder.

Comparative Example 8

An ultrafine Ni powder-terpineol slurry was obtained in the same manneras that in Example 1 except that polyoxyethylene alkyl ether (Emulgen707 manufactured by Kao Corporation) was used as the surfactant.

Comparative Example 9

An ultrafine Ni powder-terpineol slurry was obtained in the same manneras that in Example 1 except that sorbitan aliphatic acid ester (RheodolSP-030 manufactured by Kao Corporation) was used as the surfactant.

For the slurries thus obtained in Examples 2 to 32, and 35 andComparative Examples 1 to 9, by the same methods as those in Example 1,the solvent substitution, water content, dispersibility (particle sizedistribution (D90)), and dry film density were measured for evaluation.The results are shown in Table 2.

From the results shown in Table 2, compared to the ultrafine metalpowder slurries obtained in Comparative Examples 1 to 9, the ultrafinemetal powder slurries obtained in Examples 1 to 32, and 35 are superiorin terms of the solvent substitution and the dispersibility and are alsosuperior in terms of the dry film density of the conductive paste.

Next, in order to confirm the dispersibility as a conductive paste andthe reduction in labor in a process for forming a conductive paste,conductive pastes were formed from the ultrafine metal powder slurriesobtained in accordance whit the present invention.

Example 33

First, 10 parts by mass of a binder resin solution of terpineolcontaining 12 percent by mass of ethyl cellulose was added to 100 partsby mass of the ultrafine Ni powder-terpineol slurry obtained in Example1, followed by mixing for 30 minutes by an agitator. Subsequently,viscosity adjustment was performed so that the content of the ultrafineNi powder was approximately 80 percent by mass, thereby forming aconductive paste.

Example 34

A conductive paste was obtained in the same manner as that in Example 1except that the ultrafine Cu powder-terpineol slurry obtained in Example11 was used.

Comparative Example 10

After 10 parts by mass of a binder resin solution of terpineolcontaining 12 percent by mass of ethyl cellulose was added to 100 partsby mass of the ultrafine Ni powder-terpineol slurry obtained inComparative Example 1, the mixture thus obtained was then agitated for 1hour by an agitator and was then allowed to pass through a three-rollmill five times, followed by filtration treatment using a cartridgefilter type pressure filtration device. Subsequently, viscosityadjustment was performed so that the content of the ultrafine Ni powderwas an approximately 45 percent by mass, thereby forming a conductivepaste.

For evaluation of the dispersibility of the conductive pastes obtainedin Examples 33 and 34 and comparative Example 10, the conductive pastethus formed was screen-printed by hand onto a glass substrate to have athickness of 1 to 2 μm and was then dried in a drying furnace, andsubsequently, the number of projections generated on the surface of thefilm was measured by visual inspection.

Evaluation was performed by the number of projections present in an areaof 1 cm by 1 cm, and a film having a smaller number of projections wasregarded as superior. The compositions of the conductive pastes obtainedas described above and the results of the dispersion evaluation areshown in Table 3 below.

From the results shown in Table 3, it is understood that the conductivepastes of Examples 33 and 34 are significantly superior to theconductive paste of Comparative Example 10 in terms of dispersibilitysince even when the content of the ultrafine metal powder is increased,the number of projections is remarkably small. Accordingly, it is alsounderstood that reduction in labor in the process can also be realized.

Next, the relationship between the mean particle size D50 and thedispersibility of an ultrafine metal powder was investigated, thedispersibility including D90 and the number of projections on the dryfilm surface.

Examples 40 to 48

Ultrafine Ni powder-terpineol slurries were formed in the same manner asthat in Example 1 except that ultrafine Ni powders having various meanparticle diameters D50 from 0.13 to 1.15 μm were used as the ultrafinemetal powder, and that 0.3 parts by mass of oleoyl sarcosine(Sarcosinate OH manufactured by Nikko Chemical Co., Ltd.) was addedrelative to 100 parts by mass of the ultrafine Ni powder.

As the evaluation other than D90, the ultrafine Ni powder-terpineolslurry thus formed was applied using an applicator onto a glasssubstrate to have a thickness of 1 to 2 μm and was then dried in adrying furnace, and subsequently, the number of projections generated onthe surface of the film thus dried was measured. Evaluation wasperformed by the number of projections present in an area of 1 cm by 1cm, and a film having a smaller number of projections was regarded assuperior.

Evaluation Criteria of Dispersibility

The number of projections:

-   -   more than 10 . . . x (Poor)    -   5 to less than 10 . . . Δ (Fair)    -   less than 5 . . . ◯ (Good)

When the mean particle diameter D50 was in the range of 0.13 to 0.96 μm,the dispersibility D90 was particularly superior, such as less than 1.2μm. In addition, when the mean particle diameter D50 was in the range of0.61 to 0.96 μm, the number of projections was decreased to 4 or less,and the dispersibility was further improved.

TABLE 1 EXPERIMENTAL EXPERIMENTAL EXPERIMENTAL EXPERIMENTAL EXAMPLE 1EXAMPLE 2 EXAMPLE 3 EXAMPLE 4 TYPE OF SURFACTANT OLEOYL CARBOXYLATEDPOLYOXYETHYLENE ALKYLBENZENE- SARCOSINE POLYOXYETHYLENE LAURYL ETHERSULFONIC ALKYL ETHER ACETATE ACID TYPE OF ULTRAFINE METAL Ni POWDER MEANPARTICLE DIAMETER OF 0.4 μm ULTRAFINE METAL POWDER CONTENT OF ULTRAFINEMETAL 90 POWDER (MASS %) CONTENT OF 50 ULTRAFINE METAL POWDER (VOLUME %)TYPE OF ORGANIC SOLVENT TERPINEOL VISCOSITY OF 54 ORGANIC SOLVENT (cp,20° C.) VISCOSITY OF METAL 18000 30000 30000 30000 SLURRY (cps) (STRAINRATE: 1.1 sec⁻¹, 10 rpm) SOLVENT SUBSTITUTION ∘ good ∘ good ∘ good ∘good WATER CONTENT (MASS %) 0.2 0.2 0.2 0.2 DISPERSIBILITY ∘ good ∘ good∘ good ∘ good DRY FILM DENSITY (g/cm³) 5.9 5.6 5.6 5.6 REMARKS EXAMPLE 1COMPARATIVE COMPARATIVE COMPARATIVE EXAMPLE 5 EXAMPLE 6 EXAMPLE 7

TABLE 2 CONTENT OF CONTENT OF TYPE TYPE OF SURFACTANT ULTRAFINE OFORGANIC (PARTS BY METAL POWDER METAL SOLVENT TYPE OF SURFACTANT MASS)(MASS %) EXAMPLE 1 Ni TERPINEOL OLEOYL SARCOSINE 0.3 90 EXAMPLE 2 Ni 0.190 EXAMPLE 3 Ni 1.0 90 EXAMPLE 4 Ni 1.9 90 EXAMPLE 5 Ni 0.3 80 EXAMPLE 6Ni 0.3 95 EXAMPLE 7 Ni 0.3 70 EXAMPLE 8 Ni n-DODECAN 0.3 90 EXAMPLE 9 NiDIHYDROTERPINEOL 0.3 90 EXAMPLE 10 Ni TERPINEOLACETATE 0.3 90 EXAMPLE 11Cu TERPINEOL 0.3 90 EXAMPLE 12 Ag 0.3 90 EXAMPLE 13 Mo 0.3 90 EXAMPLE 14W 0.3 90 EXAMPLE 15 Co 0.3 90 EXAMPLE 16 Ta 0.3 90 EXAMPLE 17 Ni—VTERPINEOL OLEOYL SARCOSINE 0.3 90 EXAMPLE 18 Ni—Cr 0.3 90 EXAMPLE 19Ni—Nb 0.3 90 EXAMPLE 20 Ni—Mo 0.3 90 EXAMPLE 21 Ni—Ta 0.3 80 EXAMPLE 22Ni—W 0.3 95 EXAMPLE 23 Ni—Zr 0.3 70 EXAMPLE 24 Ni—Y 0.3 90 EXAMPLE 25Ni—La 0.3 90 EXAMPLE 26 Ni—Mg 0.3 90 EXAMPLE 27 Ni—Ti 0.3 90 EXAMPLE 28Ni—Ba 0.3 90 EXAMPLE 29 Ni—Ca 0.3 90 EXAMPLE 30 Ni—W—Ca 0.3 90 EXAMPLE31 Ni—Mg—Zr 0.3 90 EXAMPLE 32 Ni—Mo—Mn 0.3 90 EXAMPLE 35 Ni 0.06 90COMPARATIVE Ni TERPINEOL OLEOYL SARCOSINE 0.5 50 EXAMPLE 1 COMPARATIVENi OLEOYL SARCOSINE 0.5 97 EXAMPLE 2 COMPARATIVE Ni OLEOYL SARCOSINE0.04 90 EXAMPLE 3 COMPARATIVE Ni OLEOYL SARCOSINE 0.01 90 EXAMPLE 4COMPARATIVE Ni CARBOXYLATED 0.3 90 EXAMPLE 5 POLYOXYETHYLENE ALKYL ETHERCOMPARATIVE Ni POLYOXYETHYLENE 0.3 90 EXAMPLE 6 LAURYL ETHER ACETATECOMPARATIVE Ni ALKYLBENZENESULFONIC 0.3 90 EXAMPLE 7 ACID COMPARATIVE NiPOLYOXYETHYLENE 0.5 90 EXAMPLE 8 ALKYL ETHER COMPARATIVE Ni SORBITANFATTY ACID 0.5 90 EXAMPLE 9 ESTER SOLVENT WATER CONTENT DRY FILM DENSITYSUBSTITUTION (MASS %) DISPERSIBILITY (g/cm³) EXAMPLE 1 ∘ good 0.2 ∘ good5.9 EXAMPLE 2 ∘ good 0.2 ∘ good 5.8 EXAMPLE 3 ∘ good 0.2 ∘ good 5.9EXAMPLE 4 ∘ good 0.2 ∘ good 5.9 EXAMPLE 5 ∘ good 0.2 ∘ good 5.8 EXAMPLE6 ∘ good 0.2 ∘ good 5.8 EXAMPLE 7 ∘ good 0.2 ∘ good 5.8 EXAMPLE 8 ∘ good0.2 ∘ good 5.9 EXAMPLE 9 ∘ good 0.2 ∘ good 5.9 EXAMPLE 10 ∘ good 0.2 ∘good 5.9 EXAMPLE 11 ∘ good 0.2 ∘ good 5.9 EXAMPLE 12 ∘ good 0.2 ∘ good5.9 EXAMPLE 13 ∘ good 0.2 ∘ good 5.9 EXAMPLE 14 ∘ good 0.2 ∘ good 5.9EXAMPLE 15 ∘ good 0.2 ∘ good 5.9 EXAMPLE 16 ∘ good 0.2 ∘ good 5.9EXAMPLE 17 ∘ good 0.2 ∘ good 5.9 EXAMPLE 18 ∘ good 0.2 ∘ good 5.9EXAMPLE 19 ∘ good 0.2 ∘ good 5.9 EXAMPLE 20 ∘ good 0.2 ∘ good 5.9EXAMPLE 21 ∘ good 0.2 ∘ good 5.9 EXAMPLE 22 ∘ good 0.2 ∘ good 5.9EXAMPLE 23 ∘ good 0.2 ∘ good 5.9 EXAMPLE 24 ∘ good 0.2 ∘ good 5.9EXAMPLE 25 ∘ good 0.2 ∘ good 5.9 EXAMPLE 26 ∘ good 0.2 ∘ good 5.9EXAMPLE 27 ∘ good 0.2 ∘ good 5.9 EXAMPLE 28 ∘ good 0.2 ∘ good 5.9EXAMPLE 29 ∘ good 0.2 ∘ good 5.9 EXAMPLE 30 ∘ good 0.2 ∘ good 5.9EXAMPLE 31 ∘ good 0.2 ∘ good 5.9 EXAMPLE 32 ∘ good 0.2 ∘ good 5.9EXAMPLE 35 ∘ good 0.2 ∘ good 5.8 COMPARATIVE ∘ good 0.2 x bad 5.3EXAMPLE 1 COMPARATIVE Δ fair 0.8 x bad 5.4 EXAMPLE 2 COMPARATIVE Δ fair2 x bad 5.5 EXAMPLE 3 COMPARATIVE Δ fair 1.2 x bad 5.5 EXAMPLE 4COMPARATIVE ∘ good 0.2 ∘ good 5.6 EXAMPLE 5 COMPARATIVE ∘ good 0.2 ∘good 5.6 EXAMPLE 6 COMPARATIVE ∘ good 0.2 ∘ good 5.6 EXAMPLE 7COMPARATIVE x bad 50 x bad NO MEASUREMENT EXAMPLE 8 COMPARATIVE x bad 50x bad NO MEASUREMENT EXAMPLE 9

TABLE 3 EXAMPLE 33 EXAMPLE 34 COMPARATIVE EXAMPLE 10 TYPE OF ULTRAFINEMETAL POWDER Ni Cu Ni CONTENT OF ULTRAFINE METAL POWDER (MASS %) 81.681.6 45.6 ORGANIC SOLVENT (TERPINEOL) (MASS %) 17 17 53 CONTENT OFBINDER RESIN (ETHYL CELLULOSE) (MASS %) 1.1 1.1 1 CONTENT OF SURFACTANT(PARTS BY MASS) 0.3 0.3 0.5 DISPERSIBILITY (NUMBER OF PROJECTIONS) 1 120

TABLE 4 CONTENT CON- SOL- DISPERS- MEAN OF SUR- TENT VENT IBILITY TYPEPARTICLE FACTANT OF Ni SUB- D90 WATER DRY FILM OF DIAM- (PARTS (MASSSTITU- (MEASURED NUMBER OF CONTENT DENSITY METAL ETER/μm BY MASS) %)TION VALUE) PROJECTIONS (MASS %) (g/cm³) EXAMPLE 40 Ni 0.13 0.3 90 ∘ ∘good (0.88) ∘ good (1) 0.2 5.8 EXAMPLE 41 Ni 0.19 0.3 90 ∘ ∘ good (0.78)∘ good (1) 0.2 5.8 EXAMPLE 42 Ni 0.26 0.3 90 ∘ ∘ good (0.84) ∘ good (1)0.2 5.9 EXAMPLE 43 Ni 0.41 0.3 90 ∘ ∘ good (1.05) ∘ good (1) 0.2 5.9EXAMPLE 44 Ni 0.61 0.3 90 ∘ ∘ good (1.13) ∘ good (1) 0.2 5.8 EXAMPLE 45Ni 0.84 0.3 90 ∘ ∘ good (1.18) Δ fair (7) 0.2 5.8 EXAMPLE 46 Ni 0.96 0.390 ∘ ∘ good (1.19) Δ fair (8) 0.2 5.8 EXAMPLE 47 Ni 1.05 0.3 90 ∘ Δ fair(1.30) Δ fair (9) 0.2 5.8 EXAMPLE 48 Ni 1.15 0.3 90 ∘ Δ fair (1.35) Δfair (9) 0.2 5.8

1. An ultrafine metal powder slurry comprising: an organic solvent; asurfactant; and an ultrafine metal powder, wherein the surfactantcomprises non-neutralized acid oleoyl sarcosine, the content of theultrafine metal powder is in the range of 70 to 95 percent by mass, andthe content of the surfactant is in a range of more than 0.05 to lessthan 2 parts by mass relative to 100 parts by mass of the ultrafinemetal powder, and wherein the ultrafine metal powder comprises a nickelalloy containing nickel and at least one of vanadium, niobium,molybdenum, tantalum, tungsten, zirconium, yttrium, lanthanum,magnesium, titanium, barium, and calcium.
 2. An ultrafine metal powderslurry comprising: 70 to 95 percent by mass of an ultrafine metalpowder; more than 0.05 to less than 2 parts by mass of non-neutralizedacid oleoyl sarcosine as a surfactant relative to 100 parts by mass ofthe ultrafine metal powder; and an organic solvent as a balance, whereina particle size distribution D90 of the ultrafine metal powder is lessthan 1.2 μm, and a particle size distribution D50 indicating the meanparticle diameter is in a range of 0.1 to 1.0 μm.
 3. An ultrafine metalpowder slurry comprising: an organic solvent; a surfactant; and anultrafine metal powder, wherein the surfactant comprises non-neutralizedacid oleoyl sarcosine, the content of the ultrafine metal powder is inthe range of 70 to 95 percent by mass, and the content of the surfactantis in a range of more than 0.05 to less than 2 parts by mass relative to100 parts by mass of the ultrafine metal powder, the ultrafine metalpowder slurry having a specific density of 5 to 6 g/cm³.
 4. An ultrafinemetal powder slurry comprising: an organic solvent; a surfactant; and anultrafine metal powder, wherein the surfactant comprises non-neutralizedacid oleoyl sarcosine, the content of the ultrafine metal powder is inthe range of 70 to 95 percent by mass, the content of the surfactant isin a range of more than 0.05 to less than 2 parts by mass relative to100 parts by mass of the ultrafine metal powder, a particle sizedistribution D90 of the ultrafine metal powder is less than 1.2 μm, anda particle size distribution D50 indicating the mean particle diameteris in a range of 0.1 to 1.0 μm.
 5. The ultrafine metal powder slurryaccording to claim 4, wherein the ultrafine metal powder comprises atleast one of nickel, copper, silver, molybdenum, tungsten, cobalt, andtantalum.
 6. The ultrafine metal powder slurry according to claim 4,wherein the ultrafine metal powder comprises a nickel alloy containingnickel and at least one of vanadium, niobium, molybdenum, tantalum,tungsten, zirconium, yttrium, lanthanum, magnesium, titanium, barium,and calcium.
 7. The ultrafine metal powder slurry according to claim 4having a specific density of 5 to 6 g/cm³.